Saturday, December 21, 2013

Smog in our brains

Researchers are identifying startling connections between
air pollution and decreased cognition and well-being.

That yellow haze of smog hovering over the skyline isn't
just a stain on the view. It may also leave a mark on your mind.

Researchers have known since the 1970s that high levels of
air pollution can harm both cardiovascular and respiratory health, increasing
the risk of early death from heart and lung diseases. The effect of air
pollution on cognition and mental well-being, however, has been less well
understood. Now, evidence is mounting that dirty air is bad for your brain as
well.

Over the past decade, researchers have found that high
levels of air pollution may damage children's cognitive abilities, increase
adults' risk of cognitive decline and possibly even contribute to depression.

"This should be taken seriously," says Paul Mohai,
PhD, a professor in the University of Michigan's School of Natural Resources
and the Environment who has studied the link between air pollution and academic
performance in children. "I don't think the issue has gotten the
visibility it deserves."

Cognitive connections

Most research on air pollution has focused on a type of
pollutant known as fine particulate matter. These tiny particles — 1/30th the
width of a human hair — are spewed by power plants, factories, cars and trucks.
Due to its known cardiovascular effects, particulate matter is one of six
principal pollutants for which the Environmental Protection Agency (EPA) has
established air quality standards.

It now seems likely that the harmful effects of particulate
matter go beyond vascular damage. Jennifer Weuve, MPH, ScD, an assistant
professor of internal medicine at Rush Medical College, found that older women
who had been exposed to high levels of the pollutant experienced greater
cognitive decline compared with other women their age (Archives of Internal
Medicine, 2012). Weuve's team gathered data from the Nurses' Health Study
Cognitive Cohort, a population that included more than 19,000 women across the
United States, age 70 to 81. Using the women's address history, Weuve and her
colleagues estimated their exposure to particulate matter over the previous
seven to 14 years. The researchers found that long-term exposure to high levels
of the pollution significantly worsened the women's cognitive decline, as
measured by tests of cognitive skill.

Weuve and her colleagues investigated exposure to both fine
particulate matter (the smallest particles, less than 2.5 micrometers in
diameter) and coarse particulate matter (larger particles ranging from 2.5 to
10 micrometers in size).

"The conventional wisdom is that coarse particles
aren't as important as fine particles" when it comes to human health,
Weuve says. Previous studies in animals and human cadavers had shown that the
smaller particles can more easily penetrate the body's defenses. "They can
cross from the lung to the blood and, in some cases, travel up the axon of the
olfactory nerve into the brain," she says. But Weuve's study held a
surprise. She found that exposure to both fine and coarse particulate was
associated with cognitive decline.

Weuve's results square with those of a similar study by
Melinda Power, a doctoral candidate in epidemiology and environmental health at
the Harvard School of Public Health. Power and her colleagues studied the link
between black carbon — a type of particulate matter associated with diesel
exhaust, a source of fine particles — and cognition in 680 older men in Boston
(Environmental Health Perspectives, 2011). "Black carbon is essentially
soot," Power says.

Power's team used black carbon exposure as a proxy for
measuring overall traffic-related pollution. They estimated each man's black
carbon exposure by cross-referencing their addresses with an established model
that provides daily estimates of black carbon concentrations throughout the
Boston area. Much like Weuve's results in older women, Power and colleagues
found that men exposed to high levels of black carbon had reduced cognitive
performance, equivalent to aging by about two years, as compared with men who'd
had less black carbon exposure.

But while black carbon is a convenient marker of air
pollution, it's too soon to say that it's what's causing the cognitive changes,
Power says. "The problem is there are a lot of other things associated
with traffic — noise, gases — so we can't say from this study that it's the
particulate part of the air pollution that matters."

Still, the cumulative results of these studies suggest that
air pollution deserves closer scrutiny as a risk factor for cognitive
impairment and perhaps dementia.

"Many dementias are often preceded by a long period of
cognitive decline. But we don't know very much about how to prevent or delay
dementia," Weuve says. If it turns out that air pollution does contribute
to cognitive decline and the onset of dementia, the finding could offer a
tantalizing new way to think about preventing disease. "Air pollution is
something that we can intervene on as a society at large, through technology,
regulation and policy," she says.

Young minds

Research is also finding air-pollution-related harms to
children's cognition. Shakira Franco Suglia, ScD, an assistant professor at
Boston University's School of Public Health, and colleagues followed more than
200 Boston children from birth to an average age of 10. They found that kids
exposed to greater levels of black carbon scored worse on tests of memory and
verbal and nonverbal IQ (American Journal of Epidemiology, 2008).

More recently, Frederica Perera, DrPH, at the Columbia
University Mailman School of Public Health, and colleagues followed children in
New York City from before birth to age 6 or 7. They discovered that children
who had been exposed to higher levels of urban air pollutants known as
polycyclic aromatic hydrocarbons while in utero were more likely to experience
attention problems and symptoms of anxiety and depression (Environmental Health
Perspectives, 2012). These widespread chemicals are a byproduct of burning
fossil fuels.

Meanwhile Mohai, at the University of Michigan, found that
Michigan public schools located in areas with the highest industrial pollution
levels had the lowest attendance rates and the greatest percentage of students
who failed to meet state testing standards, even after controlling for
socioeconomic differences and other confounding factors (Health Affairs, 2011).
What's worse, the researchers analyzed the distribution of the state's public
schools and found that nearly two-thirds were located in the more-polluted
areas of their districts. Only about half of states have environmental quality
policies for schools, Mohai says, "and those that do may not go far
enough. More attention needs to be given to this issue."

Although Michigan and Massachusetts may experience areas of
poor air quality, their pollution problems pale in comparison to those of
Mexico City, for example. In a series of studies, Lilian Calderón-Garcidueñas,
MD, PhD, a neuropathologist at the University of Montana and the National
Institute of Pediatrics in Mexico City, has investigated the neurological
effects of the city's infamous smog.

In early investigations, Calderón-Garcidueñas dissected the
brains of dogs that had been exposed to air pollution of Mexico City and
compared them with the brains of dogs from a less-polluted city. She found the
Mexico City dogs' brains showed increased inflammation and pathology including
amyloid plaques and neurofibrillary tangles, clumps of proteins that serve as a
primary marker for Alzheimer's disease in humans (Toxicologic Pathology, 2003).

In follow-up research, Calderón-Garcidueñas turned her
attention to Mexico's children. In one study, she examined 55 kids from Mexico
City and 18 from the less-polluted city of Polotitlán. Magnetic resonance
imagining scans revealed that the children exposed to urban pollution were
significantly more likely to have brain inflammation and damaged tissue in the
prefrontal cortex. Neuroinflammation, Calderón-Garcidueñas explains, disrupts
the blood-brain barrier and is a key factor in many central nervous system
disorders, including Alzheimer's disease and Parkinson's disease. Perhaps more
troubling, though, the differences between the two groups of children weren't
just anatomical. Compared with kids from cleaner Polotitlán, the Mexico City
children scored lower on tests of memory, cognition and intelligence (Brain and
Cognition, 2008).

Brain changes

It's becoming clearer that air pollution affects the brain,
but plenty of questions remain. Randy Nelson, PhD, a professor of neuroscience
at the Ohio State University, is using mouse studies to find some answers. With
his doctoral student Laura Fonken and colleagues, he exposed mice to high
levels of fine particulate air pollution five times a week, eight hours a day,
to mimic the exposure a human commuter might receive if he or she lived in the
suburbs and worked in a smoggy city (Molecular Psychiatry, 2011). After 10
months, they found that the mice that had been exposed to polluted air took
longer to learn a maze task and made more mistakes than mice that had not
breathed in the pollution.

Nelson also found that the pollutant-exposed mice showed
signs of the rodent equivalent of depression. Mice said to express
depressive-like symptoms give up swimming more quickly in a forced swim test
and stop sipping sugar water that they normally find attractive. Both behaviors
can be reversed with antidepressants. Nelson found that mice exposed to the
polluted air scored higher on tests of depressive-like responses.

To find out more about the underlying cause of those
behavioral changes, Nelson compared the brains of mice that had been exposed to
dirty air with brains of mice that hadn't. He found a number of striking
differences. For starters, mice exposed to particulate matter had increased
levels of cytokines in the brain. (Cytokines are cell-signaling molecules that
regulate the body's inflammatory response.) That wasn't entirely surprising,
since previous studies investigating the cardiovascular effects of air
pollution on mice had found widespread bodily inflammation in mice exposed to
the pollution.

More surprisingly, Nelson also discovered physical changes
to the nerve cells in the mouse hippocampus, a region known to play a role in
spatial memory. Exposed mice had fewer spines on the tips of the neurons in
this brain region. "Those [spines] form the connections to other
cells," Nelson says. "So you have less dendritic complexity, and
that's usually correlated with a poorer memory."

The changes are alarming and surprising, he says. "I
never thought we'd actually see changes in brain structure."

Nelson's mice experienced quite high levels of pollution, on
par with those seen in places such as Mexico City and Beijing, which rank
higher on the pollution scale than U.S. cities. It's not yet clear whether the
same changes would occur in mice exposed to pollution levels more typical of
American cities. Another limitation, he notes, is that the animals in his study
were genetically identical. Nelson says he'd like to see similar studies of
wild-type mice to help tease out whether genetic differences might make some
people more or less vulnerable to the effects of pollution. "I would
suspect there are people who are wildly susceptible to this and people who are
less so, or not at all," he says.

Few studies have investigated connections between depression
and air pollution, but Nelson's wasn't the first. A study by Portuguese
researchers explored the relationship between psychological health and living
in industrial areas. They found that people who lived in areas associated with
greater levels of air pollution scored higher on tests of anxiety and
depression (Journal of Environmental Psychology, 2011).

Back in Ohio, Nelson plans to study how much — or how little
— pollution is necessary to cause changes in the brain and behavior. He's also
beginning to look at the effects of air pollution on pregnant mice and their
offspring. Though more research is needed to fully understand how dirty air
impairs the brain, he says, the picture that's emerging suggests reason for
concern.

In the United States, the Environmental Protection Agency
reviews the scientific basis for particulate matter standards every five years
or so, and completed its last review in 2009.

To date, the EPA hasn't factored psychological research into
their standards assessments, but that could change, according to a statement
the EPA provided to the Monitor. "Additional research is necessary to
assess the impact of ambient air pollutants on central nervous system function,
such as cognitive processes, especially during critical windows of brain development.
To this end, as the number of … studies continue to increase and add to the
weight of overall evidence, future National Ambient Air Quality Standards
assessments will again assess and address the adequacy of existing
standards."

In the meantime, says Weuve, there's not much people can do
to protect themselves, short of wearing special masks, installing special
filtration systems in their homes and offices or moving to cities with less
airborne pollution. "Ultimately, we're at the mercy of policy," she
says.

The good news, Nelson says, is that the mental
and cognitive effects of air pollution are finally beginning to receive
attention from the mental health research community. "We sort of forget
about these environmental insults," says Nelson. "Maybe we
shouldn't."

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